3. Cohort 16 only: Healthy elderly or participants with MCI or mild AD, who must have Mini Mental State Examination (MMSE) score of 18 to 30, inclusive or 18 to 26 inclusive, respectively, and no biomarker data to contradict this diagnosis. Participants with documented diagnosis of MCI or mild AD must be receiving ongoing donepezil therapy (10 mg) in the evening for a minimum of 21 days prior to Check-in (Day -1) or must consent to take donepezil dose titrated to at least 21 days of treatment with 10 mg QD prior to Check-in.